1. Field of the Invention
This invention relates to polyvinyl alcohol microgels and their use as paper and paperboard sizes, in paper and paperboard coatings, as textile warp sizes, as adhesives, and as soil and dust stabilizers.
2. Description of the Prior Art
Certain materials such as starch, sodium carboxymethyl cellulose, sodium alginate, and polyvinyl alcohol, with or without the addition of clay, have been used as surface sizes for paper and paperboard. By "surface size" is meant a material which is applied to the surface of the paper or paperboard, thereby forming a barrier which prevents or retards wicking and absorption of ink and other fluids into the paper or paperboard. Surface sizes are to be distinguished from internal sizes which are added in the process of making the sheet.
Although polyvinyl alcohol is many times more expensive than starch on a per pound basis, it has been shown that polyvinyl alcohol can perform as well as, or in many cases, better than starch as a surface size for paper or paperboard on an equal cost basis. However, in most use areas the improvement in performance has not been sufficient to cause former starch users to switch to polyvinyl alcohol. At present polyvinyl alcohol is definitely preferred to starch only in specialty areas where quality is important.
There is evidence that polyvinyl alcohol solutions readily penetrate into paper and paperboard. It is felt that even better performance for polyvinyl alcohol would be demonstrated if its "hold out" characteristics could be improved; that is, if penetration of polyvinyl alcohol size solutions into paper and paperboard could be substantially reduced or eliminated.
In a copending application of Alden J. Deyrup, Ser. No. 259,552, filed June 5, 1972, polyvinyl alcohol microgels are described which exhibit improved hold out characteristics over aqueous polyvinyl alcohol solutions. These microgels are stable fluids comprising water and polyvinyl alcohol partially crosslinked with tetravalent titanium ions and having a Brookfield viscosity of about 1.15 to 2000 centipoises.
Deyrup U.S. Pat. No. 3,492,250, in Example 4, discloses in a solution containing polyvinyl alcohol the presence of a polysaccharide thickener (Abbott Laboratories B 1459) together with Congo Red as the gelling agent. The solution of Example 4 does not contain a tetravalent titanium compound and the polysaccharide is a xanthan gum as disclosed in Whistler and Be Miller, Industrial Gums, second edition, Academic Press, New York, VII, 1. History on page 486, 1973. A repeat of Example 4 shows that the xanthan gum polysaccharide does not crosslink with the Congo Red gelling agent present in the solution. The Brookfield viscosity at 60 rpm of the solution of Example 4 containing Congo Red was found to be 70 centipoises while the Brookfield viscosity at 60 rpm of a solution without Congo Red was found to be 73 centipoises.
Deyrup U.S. Pat. No. 3,318,856 can be distinguished in that it teaches a system wherein polyvinyl alcohol is present with two different polyvinyl alcohol gelling agents, one gelling agent reinforcing the other. A polysaccharide is not present in the system. In Example 3 of said patent, boric acid functions as the initial gelling agent. Tetravalent titanium, present as potassium titanium oxalate, is the primary gelling agent used to form the rigid polyvinyl alcohol based foam described in Example 3 which has a Brookfield viscosity in excess of 2000 centipoises. The gelled skin formed in minutes on exposure to air also has a Brookfield viscosity in excess of 2000. Deyrup U.S. Pat. No. 3,492,250 in Column 4, lines 48 to 65, explains the function of two polyvinyl alcohol gelling agents including boric acid.
There has been some investigation of the use of mixtures of starch and polyvinyl alcohol as a surface size. This investigation was designed to explore the possibility of improving the quality of starch without a disproportionate increase in cost, or in lowering the cost of using polyvinyl alcohol without a disproportionate reduction in quality. These efforts, however, have been hindered by the general incompatibility of aqueous mixtures of starch and polyvinyl alcohol.